K. L. Ekinci

Ultimate limits to inertial mass sensing based upon nanoelectromechanical systems

Nanomechanical resonators can now be realized that achieve fundamental resonance frequencies exceeding 1 GHz, with quality factors (Q) in the range 10^3<=Q<=10^5. The minuscule active masses of these devices, in conjunction with their high Qs, translate into unprecedented inertial mass sensitivities. This makes them natural candidates for a variety of mass sensing applications. Here we evaluate the ultimate mass sensitivity limits for nanomechanical resonators operating in vacuo that are imposed by a number of fundamental physical noise processes. Our analyses indicate that nanomechanical resonators offer immense potential for mass sensing—ultimately with resolution at the level of individual molecules.

Ultrasensitive nanoelectromechanical mass detection

We describe the application of nanoelectromechanical systems (NEMS) to ultrasensitive mass detection. In these experiments, a modulated flux of atoms was adsorbed upon the surface of a 32.8 MHz NEMS resonator within an ultrahigh-vacuum environment. The mass-induced resonance frequency shifts by these adsorbates were then measured to ascertain a mass sensitivity of 2.53×10^–18 g. In these initial measurements, this sensitivity is limited by the noise in the NEMS displacement transducer; the ultimate limits of the technique are set by fundamental phase noise processes. Our results and analysis indicate that mass sensing of individual molecules will be realizable with optimized NEMS devices.

Monocrystalline silicon carbide nanoelectromechanical systems

SiC is an extremely promising material for nanoelectromechanical systems given its large Youngs modulus and robust surface properties. We have patterned nanometer scale electromechanical resonators from single-crystal 3C-SiC layers grown epitaxially upon Si substrates. A surface nanomachining process is described that involves electron beam lithography followed by dry anisotropic and selective electron cyclotron resonance plasma etching steps. Measurements on a representative family of the resulting devices demonstrate that, for a given geometry, nanometer-scale SiC resonators are capable of yielding substantially higher frequencies than GaAs and Si resonators.

Balanced electronic detection of displacement in nanoelectromechanical systems

We describe a broadband radio frequency balanced bridge technique for electronic detection of displacement in nanoelectromechanical systems (NEMS). With its two-port actuation-detection configuration, this approach generates a background-nulled electromotive force in a dc magnetic field that is proportional to the displacement of the NEMS resonator. We demonstrate the effectiveness of the technique by detecting small impedance changes originating from NEMS electromechanical resonances that are accompanied by large static background impedances at very high frequencies. This technique allows the study of important experimental systems such as doped semiconductor NEMS and may provide benefits to other high frequency displacement transduction circuits.

High-frequency nanofluidics: an experimental study using nanomechanical resonators.

Here we apply nanomechanical resonators to the study of oscillatory fluid dynamics. A high-resonance-frequency nanomechanical resonator generates a rapidly oscillating flow in a surrounding gaseous environment; the nature of the flow is studied through the flow-resonator interaction. Over the broad frequency and pressure range explored, we observe signs of a transition from Newtonian to non-Newtonian flow at omega tau approximately 1, where tau is a properly defined fluid relaxation time. The obtained experimental data appear to be in close quantitative agreement with a theory that predicts a purely elastic fluid response as omega tau --> infinity.

Radio-frequency scanning tunnelling microscopy

The scanning tunnelling microscope (STM) relies on localized electron tunnelling between a sharp probe tip and a conducting sample to attain atomic-scale spatial resolution. In the 25-year period since its invention, the STM has helped uncover a wealth of phenomena in diverse physical systems—ranging from semiconductors to superconductors to atomic and molecular nanosystems. A severe limitation in scanning tunnelling microscopy is the low temporal resolution, originating from the diminished high-frequency response of the tunnel current readout circuitry. Here we overcome this limitation by measuring the reflection from a resonant inductor–capacitor circuit in which the tunnel junction is embedded, and demonstrate electronic bandwidths as high as 10 MHz. This ∼100-fold bandwidth improvement on the state of the art translates into fast surface topography as well as delicate measurements in mesoscopic electronics and mechanics. Broadband noise measurements across the tunnel junction using this radio-frequency STM have allowed us to perform thermometry at the nanometre scale. Furthermore, we have detected high-frequency mechanical motion with a sensitivity approaching ∼15 fm Hz-1/2. This sensitivity is on par with the highest available from nanoscale optical and electrical displacement detection techniques, and the radio-frequency STM is expected to be capable of quantum-limited position measurements.

Diffraction effects in optical interferometric displacement detection in nanoelectromechanical systems

Optical interferometric displacement detection techniques have recently found use in the study of nanoelectromechanical systems (NEMS). Here, we study the effectiveness of these techniques as the relevant NEMS dimensions are reduced beyond the optical wavelength used. We first demonstrate that optical cavities formed in the sacrificial gaps of subwavelength NEMS enable enhanced displacement detection sensitivity. In a second set of measurements, we show that the displacement sensitivity of conventional path-stabilized Michelson interferometry degrades rapidly beyond the diffraction limit. Both experiments are consistent with numerical models.

Optical knife-edge technique for nanomechanical displacement detection

We describe an optical knife-edge technique for nanomechanical displacement detection. Here, one carefully focuses a laser spot on a moving edge and monitors the reflected power as the edge is displaced sideways. To demonstrate nanomechanical displacement detection using the knife-edge technique, we have measured in-plane resonances of nanometer scale doubly clamped beams. The obtained displacement sensitivity is in the ∼1pm∕Hz range—in close agreement with a simple analytical model.

Analysis of optical interferometric displacement detection in nanoelectromechanical systems

Optical interferometry has found recent use in the detection of nanometer scale displacements of nanoelectromechanical systems (NEMS). At the reduced length scale of NEMS, these measurements are strongly affected by the diffraction of light. Here, we present a rigorous numerical model of optical interferometric displacement detection in NEMS. Our model combines finite element methods with Fourier optics to determine the electromagnetic field in the near-field region of the NEMS and to propagate this field to a detector in the far field. The noise analysis based upon this model allows us to elucidate the displacement sensitivity limits of optical interferometry as a function of device dimensions as well as important optical parameters. Our results may provide benefits for the design of next generation, improved optical NEMS.

Multiplexed Optical Operation of Distributed Nanoelectromechanical Systems Arrays

We report a versatile all optical technique to excite and read-out a distributed nanoelectromechanical systems (NEMS) array. The NEMS array is driven by a distributed, intensity modulated optical pump through the photothermal effect. The ensuing vibrational response of the array is multiplexed onto a single probe beam in the form of a high frequency phase modulation. The phase modulation is optically down converted to a low frequency intensity modulation using an adaptive full-field interferometer, and subsequently detected using a CCD array. Rapid and single step mechanical characterization of ∼44 nominally identical high-frequency resonators is demonstrated. The technique may enable sensitivity improvements over single NEMS resonators by averaging signals coming from a multitude of devices in the array. In addition, the diffraction limited spatial resolution may allow for position-dependent read-out of NEMS sensor chips for sensing multiple analytes or spatially inhomogeneous forces.